Hitherto, various nonwoven fabrics are proposed as a material for an air filter or a liquid filter for removing dust. Particularly in recent years, long fiber nonwoven fabrics of a thermo compression bonding type having excellent rigidity are suitably used as a pleat-shaped filter. When the pleat-shaped filter is used, since a filtration area can be broad, an air velocity of filtration can be reduced, and therefore there are advantages that dust collection capacity can be enhanced or mechanical pressure drop can be reduced.
However, conventional long fiber nonwoven fabrics of a thermo compression bonding type were not a nonwoven fabric having an excellent balance between dust collection efficiency and a pressure drop and enough rigidity to be processed into a pleated configuration. The reason for this is that, in the prior art, a fiber diameter of a fiber composing the nonwoven fabric is more than about 10 μm and the nonwoven fabric had a design philosophy of controlling the dust collection efficiency and the pressure drop of the nonwoven fabric by only a degree of an opening based on this fiber diameter.
For example in Patent Document 1, a composite long fiber nonwoven fabric for filters comprising deformed fibers is proposed. It is said that according to this technology, mechanical characteristics and dimensional stability of the nonwoven fabric for filters can be improved, but a fiber diameter of a fiber composing the nonwoven fabric is 2 to 15 decitexes, that is, at least about 13 μm. Further, this technology is a technology in which by deforming a cross section of the fiber, a surface area per unit weight of fibers is increased and a contact area with dust is increased, and thereby, collection efficiency is improved. When deformed fibers are employed in melt-spinning, generally, a degree of deforming of the fiber is not so high because of the flowage of a polymer during melt-extrusion. Therefore, this nonwoven fabric could not be expected to increase a surface area considerably and could not adequately collect dust having a particle diameter of several microns or less.
Furthermore, in Patent Document 2, a nonwoven fabric for filters formed by laminating a plurality of nonwoven fabrics is proposed. It is conceivable that according to this technology, a nonwoven fabric for filters having high weight per unit area can be readily produced and a nonwoven fabric for filters having excellent air permeability can also be prepared. However, the nonwoven fabric proposed in this technology is formed by laminating and unifying a nonwoven fabric having a fiber diameter of 7 to 20 μm and a nonwoven fabric having a fiber diameter of 20 to 50 μm, and therefore this nonwoven fabric could not adequately collect dust having a particle diameter of several microns or less as with the nonwoven fabric of Patent Document 1. Furthermore, after a plurality of nonwoven fabrics are produced once, processing of laminating/unifying these nonwoven fabrics is required in addition, and therefore the productivity of this method was not high.
Further, in Patent Document 3, a nonwoven fabric for filters formed by attaching a binder resin to a long fiber nonwoven fabric formed by partial heat press bonding is proposed. According to this technology, it is conceivable that a nonwoven fabric for filters having an excellent ability of the filter to be processed into a pleated configuration and hardly producing fuzz can be obtained, but processing cost becomes very high since this nonwoven fabric needs processing of providing a binder resin. Furthermore, since the binder resin fills in voids between constituent fibers, dust collection efficiency is deteriorated and pressure drop increases.
Furthermore, in Patent Document 4, a nonwoven fabric for filters formed by fusing together fibers in the surface layer of a long fiber nonwoven fabric formed by partial heat press bonding is proposed. It is said that according to this technology, a nonwoven fabric for filters having less occurrence of fuzz even with long-term filter use can be obtained, but generally, when thermofusing of constituent fibers in the surface layer is accelerated, a fusing part increases, and when a surface area of fibers is decreased, a contact area with dust is reduced and collection efficiency is deteriorated. Furthermore, in this production step, after the filter is partially thermocompression bonded with an embossing roll, fibers in the surface layer are fused together with a flat roll, but if such the fused fibers increase, problems that the surface area of fibers for collecting dust is reduced and a pressure drop is increased arise. Further, when partial thermocompression bonding is performed in first, a sheet is partially thermocompression bonded at once and therefore bonding is possible only when the thermocompression bonding is performed at elevated temperatures and at a high line pressure. In such a case, the sheet is crushed and a nonwoven fabric having a preferable configuration for a filter cannot be attained. Moreover, there is a problem that the pressure drop increases if fibers fuse together and voids between fibers become less, and therefore this nonwoven fabric for filters was not superior in the dust collection efficiency and the pressure drop.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-276529    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-124317    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-111337    Patent Document 4: Japanese Unexamined Patent Publication No. 2005-7268